This application claims priority to Korean Patent Application No. 10-2006-0032318, filed on Apr. 10, 2006, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference.
1. Field of the Invention
The present invention relates to a multilayered white light emitting diode (“LED”) using quantum dots and a method of fabricating the same, and more particularly, to a multilayered white LED, in which a phosphor mixture layer composed of a green phosphor and a blue phosphor and a red quantum dot layer are sequentially formed on an ultra violet (“UV”) LED such that quantum dots are excited by green and blue visible light, thereby increasing luminescence efficiency and realizing stable white light, and to a method of fabricating such a multilayered white LED.
2. Description of the Related Art
A white LED using a semiconductor, having advantages such as a long lifetime, a probability of decreasing the size thereof and operability at low voltage, is receiving attention as a next-generation LED capable of replacing a conventional LED.
In order to fabricate such a white LED, a method of using three-color (e.g., red, green, blue) LEDs has been conventionally proposed, but suffers because it results in a high fabrication cost and a large product size due to the complicated driving circuit.
Further, a white LED is realized in practice by combining an InGaN blue LED having a wavelength of 450 nm with a YAG:Ce phosphor, based on the principle in which part of the blue light generated from the blue LED excites the YAG:Ce phosphor to generate yellow-green light, which is then mixed with the blue light, thus emitting white light.
However, since light of the white LED, resulting from the combination of the blue LED and the YAG:Ce phosphor, has only part of the visible light spectrum, it has a low color rendering index and thus does not exhibit a desired color. Further, due to the 450 nm wavelength of the blue LED used as an excitation source, chip efficiency is decreased, leading to decreased overall luminescence efficiency of the white LED.
In order to solve the problems associated with the white LED, thorough attempts have been made to develop a white LED capable of emitting a white color similar to natural light by mixing red, green and blue phosphors using a UV LED as an excitation source.
That is, as shown in FIG. 1, there has been proposed a conventional technique of realizing white light by applying red, green and blue phosphors around a UV short-wavelength LED such that the red, green and blue phosphors are excited by the UV LED, having high energy, and thus respectively emit red, green and blue wavelengths. The red, green and blue phosphors emitting red, green and blue wavelengths are then mixed.
However, according to such a conventional technique, the properties of the white LED may be deteriorated, depending on phosphor conversion efficiency, self absorption, temperature, and use for a long period of time. In particular, the properties of the white LED are limited due to the low phosphor conversion efficiency of the red phosphor.
With the intention of solving the problems associated with the white LED using the above phosphors, a white quantum dot and colored LED are disclosed in U.S. Pat. No. 6,890,777 and Japanese Patent Laid-open Publication No. 2002-510899.
The white quantum dot and colored LED are composed of a primary light source, a host matrix and a population of quantum dots embedded in the host matrix, as shown in FIGS. 2A and 2B.
The population of the quantum dots, including a mixture of quantum dots respectively emitting light of predetermined wavelengths, may be provided in the form of a single layer, as shown in FIG. 2A, or may be provided in the form of separate layers, as shown in FIG. 2B. Such an LED may be constructed to have a large number of quantum dots, whereby substantially all light from the primary source is absorbed and the finally emitted radiation is produced only by luminescence of the quantum dots. Alternatively, such an LED may be constructed to have a smaller number of quantum dots, whereby the light emerging from the device consists of a mixture of light from the primary source unabsorbed to the quantum dots and of light produced by luminescence of the quantum dots.
However, since the quantum dots of the quantum dot LED are directly excited by UV light having high energy, leading to luminescence, such an LED cannot but have drastically decreased luminescence efficiency upon long exposure to UV light. Accordingly, the quantum dot LED is disadvantageous because it cannot be applied to devices which must be used for a long period of time.
Consequently, in the case of realizing white light using a UV high-output LED, there is a need for an LED having high luminescence efficiency and which is capable of stably maintaining white light.